In optical communication systems, avalanche photodiodes (APDs) are used to convert optical signals into electrical signals. APDs having a planar configuration are among the most reliable. With reference to FIGS. 1 and 2, a typical planar APD comprises an APD body 101 or 201 of semiconductor material, as disclosed in U.S. Pat. No. 6,515,315 to Itzler, et al., issued on Feb. 4, 2003; and in U.S. Patent Application Publication No. 2008/0121867 to Yagyu, et al., published on May 29, 2008; respectively, which are incorporated herein by reference.
The APD body 101 or 201 includes a substrate 110 or 210 and a layer stack 120 or 220, disposed on a front surface of the substrate 110 or 210. The layer stack 120 or 220 comprises an absorption layer 121 or 221 of thickness Wa, for absorbing light at an absorption wavelength band to generate a photocurrent, a multiplication layer 122 or 222, which includes a multiplication region of thickness Wm, for multiplying the photocurrent through avalanche multiplication, and a field-control layer 123 or 223, for controlling electric fields in the absorption layer 121 or 221 and the multiplication layer 122 or 222.
In the APD body 101, the multiplication layer 122 includes a diffusion region 124, for defining the multiplication region and for providing a p-n junction. The layer stack 120 further comprises a buffer layer 125, for accommodating lattice mismatch between the substrate 110 and the layer stack 120, and a grading layer 126, for facilitating current flow between the absorption layer 121 and the multiplication layer 122.
In the APD body 201, the layer stack 220 further comprises a buffer layer 225, for accommodating lattice mismatch between the substrate 210 and the layer stack 220, and a window layer 227, for transmitting light at the absorption wavelength band to the absorption layer 221. The window layer 227 and the absorption layer 221 include a diffusion region 224, for defining the multiplication region and for providing a p-n junction.
The APD body 101 or 201 may be included in an APD having a front-illuminated or a back-illuminated configuration. In either configuration, the thickness of the absorption layer 121 or 221 plays an important role in determining many characteristics of the APD, such as the breakdown voltage, the process window of the thickness of the multiplication region, and the intrinsic responsivity.
The breakdown voltage of the APD decreases linearly with decreasing thickness of the absorption layer 121 or 221. Thus, from an operational point of view, the thickness of the absorption layer 121 or 221 should be minimized to minimize the breakdown voltage, as a lower breakdown voltage allows the APD to be operated at a lower voltage.
The process window, i.e. the allowable range, of the thickness of the multiplication region in the APD increases with decreasing thickness of the absorption layer 121 or 221. Thus, from a manufacturing point of view, the thickness of the absorption layer 121 or 221 should be minimized to maximize the process window of the thickness of the multiplication region, as a larger allowable range of the thickness of the multiplication region can improve the manufacturing yield of the APD.
However, the intrinsic responsivity, i.e. the responsivity at a photocurrent gain of 1, of the APD decreases with decreasing thickness of the absorption layer 121 or 221. Thus, from a performance point of view, the thickness of the absorption layer 121 or 221 should be maximized to maximize the intrinsic responsivity, as a higher intrinsic responsivity can increase the sensitivity of the APD.
One approach to increasing the intrinsic responsivity of the APD, without increasing the breakdown voltage or decreasing the process window of the thickness of the multiplication region, is to increase the effective thickness of the absorption layer 121 or 221 by including a reflector in the APD.
For example, front-illuminated APDs including a layer stack comprising a distributed Bragg reflector (DBR) of semiconductor material have been disclosed in an article entitled “Simple Planar Structure for High-Performance AlInAs Avalanche Photodiodes” by Yagyu, et al. (IEEE Photonics Technology Letters, 2006, Vol. 18, pp. 76-78); in an article entitled “High Efficiency 10 Gbps InP/InGaAs Avalanche Photodiodes with Distributed Bragg Reflector” by Ishimura, et al. (Proceedings of the 27th European Conference on Optical Communication (ECOC '01), 2001, Vol. 4, pp. 554-555); in U.S. Pat. No. 7,259,408 to Yagyu, et al., issued on Aug. 21, 2007; in U.S. Pat. No. 7,038,251 to Ishimura, et al., issued on May 2, 2006; and in U.S. Pat. No. 5,880,489 to Funaba, et al., issued on Mar. 9, 1999.
In such front-illuminated APDs, light incident on a front surface of an APD body passes through a front portion of the layer stack to an absorption layer, where a portion of the incident light is absorbed. An unabsorbed portion of the incident light passes through a back portion of the layer stack to the DBR, where it is reflected back for a second pass through the absorption layer, increasing the effective thickness of the absorption layer and the intrinsic responsivity of the front-illuminated APDs. However, the formation of a DBR of semiconductor material with a high reflectivity is challenging and adds complexity to the manufacturing process of the front-illuminated APDs. Furthermore, the DBR has a high reflectivity at only a narrow wavelength band.
To attempt to overcome such limitations, a reflector has been included on a front surface of an APD body in back-illuminated APDs, as disclosed in U.S. Pat. No. 6,894,322 to Kwan, et al., issued on May 17, 2005; in U.S. Pat. No. 6,831,265 to Yoneda, et al., issued on Dec. 14, 2004; in U.S. Pat. No. 6,693,337 to Yoneda, et al., issued on Feb. 17, 2004; in U.S. Pat. No. 5,610,416 to Su, et al., issued on Mar. 11, 1997; and in U.S. Pat. No. 5,179,430 to Torikai, issued on Jan. 12, 1993; for example.
In such back-illuminated APDs, light incident on a back surface of the APD body passes through a substrate and a back portion of a layer stack to an absorption layer, where a portion of the incident light is absorbed. An unabsorbed portion of the incident light passes through a front portion of the layer stack to the reflector on the front surface of the APD body, where it is reflected back for a second pass through the absorption layer, increasing the effective thickness of the absorption layer and the intrinsic responsivity of the back-illuminated APDs. However, the manufacturing yield of back-illuminated APDs is relatively low, because, typically, many process steps are carried out on the back side of the APD body after the substrate is thinned by lapping. Furthermore, the package-assembly cost of back-illuminated APDs is relatively high, because the chips must, typically, be flipped to allow back illumination.
Similarly, a reflector has been included on a back or a front surface of a positive-intrinsic-negative (PIN) photodiode body in front-illuminated and back-illuminated PIN photodiodes, respectively, as disclosed in U.S. Pat. No. 6,831,265; in U.S. Pat. No. 6,693,337; and in U.S. Pat. No. 5,942,771 to Ishimura, issued on Aug. 24, 1999; for example.
The present invention provides front-illuminated APDs with improved intrinsic responsivity, which comprise an APD body having a layer structure similar to that of the prior-art APD body 101 or 201, but which have configurations and features distinct from those of prior-art photodiodes. Advantageously, a back surface of the APD body is mechanically and chemically polished to increase a reflectance of the back surface of the APD body at an absorption wavelength band. In addition, a reflector having a reflectance of greater than 90% at the absorption wavelength band is disposed on the back surface of the APD body.
Light incident on a front surface of the APD body passes through a front portion of a layer stack to an absorption layer, where a portion of the incident light is absorbed. An unabsorbed portion of the incident light passes through a back portion of the layer stack and, in some instances, a substrate to the reflector on the back surface of the APD body, where it is reflected back for a second pass through the absorption layer, increasing the effective thickness of the absorption layer and the intrinsic responsivity of the front-illuminated APDs.